1. Field of the Invention
The present invention relates to an ultrasonic sensor that has an ultrasonic detecting element mounted on an acoustic matching layer thereof.
2. Description of Related Art
There is conventionally known an ultrasonic sensor in which an ultrasonic transducer is attached to a substrate made of a metal, a resin material or the like, the ultrasonic sensor being mounted on. For example, an automobile (vehicle). The ultrasonic sensor transmits an ultrasonic wave by using an element capable of transmitting and receiving the ultrasonic wave, and receives the ultrasonic wave, which collides with and is reflected from a detected body by using the element, thereby performing a position measurement or a distance measurement of an object around the vehicle, or a measurement of a two-dimensional configuration or a three-dimensional configuration of the object.
Such an ultrasonic sensor may be provided with an acoustic matching layer, which adjusts an acoustic impedance to improve a transmission efficiency of transmitted/received ultrasonic waves. For example, there is known an ultrasonic sensor in which an acoustic matching layer is fixed on one face of a piezoelectric element, the acoustic matching layer being formed to have a thickness of one quarter of the ultrasonic wave by a material, in which glass balloons are diffused in an synthetic resin (see JP-A-H10-224895).
Because the ultrasonic sensor of the above type is attached to a place, which is observable from exterior, the ultrasonic sensor is reduced in size in order not to degrade a sense of beauty of the appearance. In other words, the area of the acoustic matching layer, the area being exposed to the exterior, is required to be made small.
However, when the area of the acoustic matching layer is reduced, the influence of the reflection of the ultrasonic wave at the side face of the acoustic matching layer becomes larger. As a result, the ratio of the noise to the signal becomes larger, and thereby the detection sensitivity to the ultrasonic wave is degraded. Due to the above disadvantage, the area of the acoustic matching layer has been kept from being made smaller.